The present invention relates to a process for the manufacture of sodium perborate monohydrate from sodium perborate tetrahydrate.
Sodium perborate of the empirical formula NaBO.sub.7 H.sub.8 ; called sodium perborate tetrahydrate, is currently being used in order to obtain washing compositions. It, however, presents characteristics such as, for instance, solution velocity and of temperature stability, which are still insufficient in order to give full satisfaction in this field of use.
Accordingly, so-called sodium perborate monohydrate of empirical formula NaBO.sub.4 H.sub.2 is becoming increasingly preferred over it. In comparison to sodium perborate tetrahydrate, the monohydrate possesses not only an active oxygen content which is greater by about 50%, but also a more rapid solution velocity and a higher melting temperature.
The known processes for the manufacture of sodium perborate monohydrate from sodium perborate tetrahydrate consist of eliminating from the latter the desired quantity of water of crystallization by means of a hot air current.
In order to ensure a good distribution of the hot air in the mass to be dehydrated and to avoid any great decomposition of the latter, it has generally been proposed to carry out its dehydration by maintaining it in the fluidized state by means of the hot air current.
This is, for instance, the case as far as the procedures described in Belgian Patent No. 718,160 and in French Patent No. 1,081,421 are concerned. The advantage of such procedures is, in fact, to ensure a uniform temperature in the mass to be dehydrated.
However, as indicated by French Patent No. 2,207,859, the dehydration of sodium perborate tetrahydrate by the hot air leads either to a sodium perborate monohydrate having an insufficient abrasion resistance or to a product having an improved abrasion resistance, but then only in a process operating at a temperature which is too high to avoid the fusion (melting) of a part of the load and the presence of "crusts" in the resultant product.
And so the object of French Patent No. 2,207,859 is to obtain a product of better abrasion resistance than that of the products resulting from prior processes with hot air, by yielding only a minimal fusing of the sodium perborate grains. This minimal fusing of the grains is made possible due to a high water vapor content of the hot air in contact with the mass to be dehydrated during the course of all or part of the operation. This high relative humidity is achieved by means of the addition of saturated water vapor to the air at the time of its introduction into the zone of dehydration.
French Addition Certificate No. 2,285,339 to French Patent No. 2,207,859 specifies that the humidity of the air necessary in order to achieve the result of the main patent can be ensured by virtue of the evaporation of the water of crystallization alone, but then it is necessary to carry out the dehydration operation in two stages, the first one of which must be slow.
The processes described in the above French patent and French Addition Certificate present the following drawbacks: causing the formation of fine particles which are carried along by the air and water vapor current and, in order that a large part of the product is not lost, necessitating the separation of these find particles from the gas current and this is an efficient manner and thus costly, and offering, at least locally, a sure risk of total fusing of the grains as a consequence of the principle itself on which these processes are based.
The known processes neither permit achieving a higher production, nor carrying out the operation in homogeneous manner without loss by decomposition, nor, when this stumbling block is avoided, leading to a product of sufficient abrasion resistance, nor, when this property is found to be improved, avoiding a consequent attrition and the risk of fusion of the grains.